A car alarm is an electronic device installed in a vehicle in an attempt to discourage theft of the vehicle itself, its contents, or both.
Car alarms work by emitting high-volume sound (usually a siren, klaxon, pre-recorded verbal warning, the vehicle's own horn, or a combination thereof)
when the conditions necessary for triggering are met, as well as by flashing some of the vehicle's lights, and (optionally) notifying the car's owner via a paging system
and interrupting various electrical circuits necessary for the car to start.
Car alarms should not be confused with immobilizers; although the purpose of both may be to deter car theft, they operate in a dissimilar fashion.
An immobilizer generally will not offer any audible or visual theft deterrence, nor require any additional input from the driver than a non-immobilizer car.
Car alarms can be designed to be triggered by vibrations, tilting o f the car (to prevent unauthorized towing), touching the car, the opening or closing of special switches
(e.g. door contacts), sensing small but rapid changes in battery voltage (which might indicate an interior light going on, or the ignition circuit being activated),
or using volumetric sensors such as ultrasound , infrared or microwave.
Many times a car alarm can be triggered accidentally. This may be caused by the passing of large trucks, the vibration of thunder or people coming into
contact with the vehicle, triggering the alarm sensors. Some sensors may need adjustment in order to prevent false alarms.